Base: Add man pages for recvfd(2) and sendfd(2) :^)

Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/1a7ccc15696

Base/usr/share/man/man2/recvfd.md

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+## Name
+
+recvfd - receive a file descriptor from a local socket peer
+
+## Synopsis
+
+```**c++
+#include <sys/socket.h>
+
+int recvfd(int sockfd);
+```
+
+## Description
+
+Receive an open file descriptor from a local socket peer connected via `sockfd`. This is a non-blocking call that will fail if there is no file descriptor waiting in the socket's queue.
+
+File descriptors are sent out-of-band and do not affect the regular data streams.
+
+## Return value
+
+If a file descriptor is successfully received, it is returned as a positive integer. Otherwise, -1 is returned and `errno` is set to indicate the error.
+
+## Errors
+
+* `EBADF`: `sockfd` is not an open file descriptor.
+* `ENOTSOCK`: `sockfd` does not refer to a socket.
+* `EAFNOSUPPORT`: `sockfd` does not refer to a local domain socket.
+* `EINVAL`: `sockfd` does not refer to a connected or accepted socket.
+* `EAGAIN`: There is no file descriptor queued on this socket.
+
+## See also
+
+* [`sendfd`(2)](sendfd.md)

Base/usr/share/man/man2/sendfd.md

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+## Name
+
+sendfd - send a file descriptor to a local socket peer
+
+## Synopsis
+
+```**c++
+#include <sys/socket.h>
+
+int sendfd(int sockfd, int fd);
+```
+
+## Description
+
+Send an open file descriptor to a local socket peer connected via `sockfd`. This is a non-blocking call that will fail if there are too many file descriptors already waiting to be received by the peer.
+
+File descriptors are sent out-of-band and do not affect the regular data streams.
+
+## Return value
+
+If a file descriptor is successfully received, it is returned as a positive integer. Otherwise, -1 is returned and `errno` is set to indicate the error.
+
+## Errors
+
+* `EBADF`: `sockfd` or `fd` is not an open file descriptor.
+* `ENOTSOCK`: `sockfd` does not refer to a socket.
+* `EAFNOSUPPORT`: `sockfd` does not refer to a local domain socket.
+* `ENOTCONN`: `sockfd` refers to a socket which is not connected.
+* `EINVAL`: `sockfd` does not refer to a connected or accepted socket.
+* `EBUSY`: There are too many file descriptors already waiting to be received by the peer.
+
+## See also
+
+* [`recvfd`(2)](recvfd.md)